Subject: unregistering tcp_ca struct_ops can cause kernel page fault

From: [email-masked]
Date: Sun, 08 Dec 2024 09:59:36 -0500

The attached program, along with the attached bpf_cubic.o eBPF binary,
uses bpftool to install a tcp_ca struct ops, creates a tcp connection,
and then unregisters the struct ops while the tcp connection is still
active. On my 6.13.0 system this causes a kernel fault due to
tcp_tso_segs() calling through a de-allocated struct tcp_congestion_ops.

bpf_cubic.o came from

  https://github.com/aroodgar/bpf-tcp-congestion-control-algorithm

Linux ubuntu66 6.13.0-rc1-00337-g7503345ac5f5 #11 SMP Sun Dec  8 08:37:57 EST 2024 x86_64 x86_64 x86_64 GNU/Linux

Oops: general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b6b: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
CPU: 6 UID: 0 PID: 1594 Comm: a.out Not tainted 6.13.0-rc1-00337-g7503345ac5f5 #11
Hardware name: FreeBSD BHYVE/BHYVE, BIOS 14.0 10/17/2021
RIP: 0010:__x86_indirect_thunk_array+0xa/0x20
Code: 66 0f 1f 00 31 ff e9 15 70 d6 fe cc cc cc cc cc cc cc cc cc cc cc cc cc cc
 cc cc cc cc cc cc cc e8 01 00 00 00 cc 48 89 04 24 <c3> cc cc cc cc 90 66 66 2e
 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00
RSP: 0018:ffffc90002037c60 EFLAGS: 00010202
RAX: 6b6b6b6b6b6b6b6b RBX: ffff88810afc0b00 RCX: 0000000000000018
RDX: 00000000077b668a RSI: 0000000000008000 RDI: ffff88810afc0b00
RBP: 0000000000008000 R08: 0000000000000000 R09: 0000000000008000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000008000 R14: 0000000000000000 R15: ffffffff842f6140
FS:  00007f5457b84740(0000) GS:ffff88842db80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f5457d8c710 CR3: 000000011b4b8005 CR4: 00000000003706f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 ? die_addr+0x31/0x80
 ? exc_general_protection+0x1b4/0x3c0
 ? asm_exc_general_protection+0x26/0x30
 ? __x86_indirect_thunk_array+0xa/0x20
 ? tcp_tso_segs+0x1c/0x90
 ? tcp_write_xmit+0x74/0x1840
 ? __mod_memcg_state+0x91/0x190
 ? __tcp_push_pending_frames+0x31/0xc0
 ? tcp_sendmsg_locked+0xafc/0xd10
 ? tcp_sendmsg+0x26/0x40
 ? sock_write_iter+0x167/0x1a0
 ? vfs_write+0x35d/0x400
 ? ksys_write+0xc6/0xe0
 ? do_syscall_64+0x3f/0xd0
 ? entry_SYSCALL_64_after_hwframe+0x76/0x7e
 </TASK>

Robert Morris
[email-masked]